Visual Glossary

Soil is more than just dirt!

Soil is all loose, unconsolidated earth and organic materials above bedrock
that support plant growth.

Given enough time, solid rock exposed at the
Earth's surface is broken
down into rock and mineral fragments
by weathering.
That's why most of the land surface is covered by a layer of loose
material that geologists
call surficial materials, but is often called dirt.

Soil
is more than just dirt! While a layer of rock and mineral
fragments is the prime ingredient, it takes decayed plant and
animal remains (organic matter), and water to turn rocky
debris into soil. Changing the proportions of these key
ingredients produces a remarkable
variety of soil types.

Rock, time, and climate

Every soil starts from a rocky parent (the technical term is parent material) and the type of parent rock has a strong influence on soil formation. Different types of rocks have different chemical makeups. As rock and minerals are broken down by weathering,
chemical reactions between minerals, water, and air may
produce new minerals such as clays. Disintegrating rock and weathering
products accumulate to
begin the soil-forming process.

Time, as always in geology, is another critical factor
in soil formation. The characteristics of a particular
soil depend largly on the lenght of time it's had to develop. When
soils first begin forming, the nature of the parent
material overwhelmingly determines the nature of the soil. Over time,
parent material becomes
more weathered and climate plays an increasingly important
role in the soil-forming process.

Climate is perhaps the most important ingredient in
soil formation. The amount of rain and other forms
of precipitation and temperature patterns influence
everything from the rate of rock weathering
to the types of plants that grow on soils. As a consequence,
desert, rain forest, and grassland soils develop
distinctly different characteristics.